This invention relates to novel copolyesters and adhesive compositions formed therefrom.
Vinyl materials often contain a significant amount of plasticizer to render them soft and flexible, the most common and least expensive plasticizer being di-2-ethylhexyl phthalate (di-octyl phthalate), although di-octyl isophthalate, di-octyl adipate, di-octyl sebacate, and other polymeric and non-polymeric plasticizers are also useful. Various problems have been associated with the use of adhesives for plasticized vinyl substrates such as vinyl wall coverings, vinyl tile, edge banding and other vinyl laminations. For instance, over a period of time the plasticizers from the highly plasticized vinyl substrate tend to migrate to the adhesive/substrate interface, and even into the adhesive itself. Attack of the adhesive interface by the plasticizer results in a rapid loss in adhesion, and attack of the adhesive itself by the plasticizer results in a loss of cohesion or internal strength of the adhesive, accompanied by splitting and transfer.
Conventional solution and hot-melt adhesives employing, for example, ethylene/vinyl acetate copolymers and rubber based adhesives are rapidly attacked by the migrating plasticizers from the vinyl substrate resulting in a decrease in the tensile peel values of the adhesive bond to very low levels (when the plasticizer migrates into the interface) and a loss of cohesive strength of the adhesive (when the plasticizer migrates into the adhesive). Accompanying the migration of plasticizer from the vinyl substrate is a rapid shrinkage of the vinyl sbustrate by as much as 5 to 10%. While the acceptable degree of vinyl shrinkage or decrease in peel strength will, of course, vary with the application, typical requirements for the adhesives utilized in connection with vinyl substrates are a 40-150 ozs./linear inch peel strength (or bond value) with less than a 30% decrease after aging for one week at 70.degree. C., and less than 0.5% vinyl shrinkage, with no adhesive splitting or transfer, after similar aging.
Additional requirements for both hot melt and solution adhesives in general are high heat resistance or heat distortion temperature (i.e., the ability to form an adhesive bond which withstands high temperatures without softening and losing adhesion and adherence) and cold resistance (i.e., the ability to form a flexible adhesive bond which withstands low temperatures without becoming brittle). Of course, in the case of solution adhesives, it is necessary also that the adhesives be applicable to a substrate from a solution, and in the case of hot melt adhesives that the adhesives have high thermal or heat stability at elevated temperatures (i.e., the ability to resist degradation or gelation at high temperatures over time). Both require good wetting characteristics for good adhesion, low viscosity for ease of application, good cohesive strength, color (preferably clear or light), and compatibility with a large variety of tackifying resins, plasticizers, solvents, modifiers, pigments and the like.
Good low temperature properties are usually associated with polymers having a low glass transition temperature, high heat resistance with crystalline or semi-crystalline polymers, and good cohesive strength with high molecular weight polymers and crystalline or semi-crystalline polymers.
Copolyesters of ethylene glycol, terephthalic acid and dimer acid and similar copolyesters have been described in various patents such as U.S. Pat. Nos. 3,383,343; 3,390,108; 3,329,740 and French Pat. No. 1,398,551. Such copolyesters have not proven to be especially useful as adhesives for vinyl substrates despite their low shrinkage levels, because they are not highly resistant to plasticizer migrating from the vinyl substrate. Typically the peel strength decreases by at least 50% on aging, with accompanying adhesive transfer and splitting. Attempts to improve the adhesive properties of such copolyesters by chain extending the copolyesters with various non-polymeric polyols such as trimethylol propane and various polyether polyols such as trimethylol propane/ethylene oxide adducts have not proven successful. Another feature inhibiting the use of such copolyesters has been the cost and difficulty in manufacturing. In conventional copolyester manufacture, high temperatures (up to 280.degree. C.), high vacuum (1.0-0.1 mm. Hg.) and long polycondensation times are required to obtain products exhibiting useful adhesive characteristics. Such extreme conditions do not always result in the best obtainable products due to degredation reactions occurring during polycondensation, and of course increase the cost of manufacture.
U.S. Pat. No. 3,975,323 issued Aug. 17, 1976, describes copolyesters of the type mentioned above which have been chain extended with a polyester polyol to increase the molecular weight thereof to provide increased plasticizer migration tolerance without loss of cohesive or adhesive properties, even at elevated termperature storage. The chain-extended copolyesters are characterized by glass transition temperature of less than 0.degree. C. and a melting temperature greater than 70.degree. C. Nonetheless, these chain-extended copolyesters have not proven to be entirely satisfactory. For instance, compatibility of the chain-extended copolymers with hydrocarbon-type solvents, tackifying resins, and non-oxidized polyethylene waxes is less than desired. Furthermore, adhesion of the chain-extended copolyesters to particular substrates such as the urea-and phenol-formaldehyde resins (available under the trade name FORMICA) is not as high as desired. Finally, many chain-extended copolyesters tend to set too fast, thereby resulting in an inadequate wetting and premature shrinkage and so weakening the adhesive bond.
Copolyesters of ethylene glycol, terephthalic acid and particular substituted succinic acid have been described in U.S. Pat. No. 3,542,737. The copolyesters are prepared using polycondensation temperatures of 275.degree. C., at which temperature succinic compositions tend to decompose (i.e., decarboxylate), thus causing chain termination and so lowering the molecular weight of the final copolyester. However, the patent teaches the use of such low percentages of the succinic constituent (0.5-15 percent based on the moles of acid units in the copolyester) that, as a matter of fact, the succinic constituent plays very little part in the overall reaction.
Accordingly, it is an object of the present invention to provide a novel copolyester which is a high molecular weight chain-extended aromatic dicarboxylic acid/aliphatic dicarboxylic acid/glycol copolyester useful as an adhesive.
Another object is to provide such an adhesive which inhibits shrinking of a vinyl substrate and is resistant to plasticizer migrating from the vinyl substrate.
A further object is to provide such an adhesive exhibiting a 40-150 ounces/linear inch peel strength with a maximum 30% decrease in strength after aging (1 week at 70.degree. C.) and a maximum 0.5% shrinkage of the vinyl substrate with no adhesive splitting or transfer after similar aging.
It is also an object to provide an adhesive exhibiting good thermal stability, high heat and cold resistance, good wetting characteristics, low viscosity, good cohesive strength, and a high compatibility with a large variety of tackifying resins, plasticizers, solvents, modifiers, pigments, etc.
It is a further object to provide a method of manufacturing such polyesters under very mild conditions involving temperatures no higher than 265.degree. C., pressures of 1-30 mm. Hg. and relatively short polycondensation times of 2-7 hours.